There are a number of visual and other functional benefits in printing only part of the area of a substrate, some of which are outlined in GB 2 118 096 (Hill & Yule), GB 2 165 292 (Hill), PCT/GB98/02600 (Hill) and PCT/GB97/00020 (Hill).
Methods of partially printing a substrate with substantially exact registration of superimposed layers are outlined in GB 2 118 096, GB 2 165 292, GB 2 188 873, PCT/GB96/02600, PCT/GB97/02788 (Hill and Godden) and PCT/IB00/00267 (Hill and Clare). However, these methods typically involve the use of special inks and additional production stages compared to the simple deposition of layers of ink or other marking material. Partially printed substrates can typically be produced more economically by conventional methods of printing having inexact registration, providing any undesirable effects of such lack of registration can be consistently overcome.
PCT/GB96/02600 discloses why conventional printing processes all suffer inexact registration, owing to:                i) printing machine error or “tolerance” in delivering ink or other marking material,        ii) the dimensional instability of a liquid ink or other marking material in liquid state on a substrate,        iii) the dimensional instability of a substrate through temperature and humidity changes between printing “passes” (printing of individual layers), and        iv) the error or “tolerance” in delivery of a substrate into the printing position.Panels according to GB 2 165 292 cannot be reliably produced with conventional printing methods without special measures to overcome the otherwise inevitable color variance from that desired, both over the area of a single panel and between panels within a production run. Two such methods of creating acceptable panels by the management of conventionally printed layers are described in PCT/GB96/02600, referred to as the “Lateral Combination Method” and the “Through Combination Method.”        
With regard to those figures illustrating a cross-section through a single printed portion on a substrate 10, it should be understood that the single printed portion is typically representative of a plurality of such printed portions on a cross-section through substrate 10.
FIGS. 1A and 1B illustrate prior art vision control panels according to GB 2 165 292, which comprise transparent substrate 10 and printed portions typically comprising black layer 50, white layer 30 and design color layer 40 superimposed with substantially exact registration, such that design color layer 40 is visible from one side of the panel but is not visible from the other side of the panel.
FIGS. 1C and 1D illustrate the “Lateral Combination Method” and FIGS. 1E and 1F illustrate the “Through Combination Method” of overcoming the inevitable lack of registration and problems that would otherwise result if it was attempted to print multiple layers of the same dimensions within printed portions by conventional means. FIG. 2A illustrates a notional arrangement in which substrate 10 has black layer 50, white layer 30 and design color layer 40 in substantially exact registration, having “desired perceived color” width 160 between the desired coterminous alignment of outer edges 80 at desired outer edge locations 81. If conventional printing methods are used to try to print the notional arrangements of FIG. 2A, because of the inevitable lack of registration, the layers will typically each be displaced laterally, for example as illustrated in FIG. 2B, in which layers 40 and 50 have moved by a dimension “t” to the left of their desired alignment and layer 30 has moved by a dimension “t” to the right of its desired alignment. As outlined in PCT/GB96/02600, such registration errors will typically cause very substantial changes from the colors that are desired to be perceived on particular parts of a single panel, and variance from panel to panel in a production run. In FIG. 2B, owing to registration error, the white layer 30 overlaps beyond the black layer 50 and thus is visible from the other side of the panel and increases the width of this printed portion, both undesirable features for panels according to GB 2 165 292. Also, if design layer 40 comprises a translucent ink, for example a single “spot” translucent color ink or the cyan, magenta, yellow and black translucent inks of a CMYK four color process design, such lack of registration on a relatively small printed portion causes dramatic variance of perceived color from that desired. Where the translucent design color layer 40 ink lies over black layer 50 it will be effectively rendered invisible by the underlying black layer. The actual perceived color width 60 of the translucent design color layer 40 ink overlying the white layer 30 is less than the desired perceived color width 100. Also, part of the white layer 30 is exposed for a width of t+t1. The actual perceived color has a reduced intensity and lightened greytone compared to that desired.
Depending on the method of printing, the equipment used, the operator's skills, the disciplines introduced to control registration, the type of substrate, the size of the area being printed and the direction of application of ink, for example the direction of a squeegee pull in screen printing, the reliably achievable registration tolerance will vary. However, for any given printing set-up, it is possible to establish a reliably achievable registration tolerance “T” in a given direction in relation to the direction of application of ink. It is typically desirable to produce products according to GB 2 188 873  GB 2 165 292 according to the “Lateral Combination Method” or “Through Combination Method” of PCT/GB96/02600 with printed portions in a pattern of lines oriented in the direction of application of ink. This is because the registration tolerance “T” is less perpendicular to the lines than the registration tolerance “T+ΔT” in the direction of application of ink. Thus, in screen printing, lines would typically be oriented in the direction of squeegee pull; in litho printing the lines would be orientated parallel to the direction of travel of the substrate through the machine.
GB 2 188 873  GB 2 165 292 identifies in “Overlap Method 1” that a design visible from one side of a panel can be obscured from visibility from the other side of a panel by the simple expedient of one layer, typically a black silhouette pattern layer according to that invention, overlapping other layers, typically one or more white background layers and one or more design color layers. FIG. 3A is a notional cross-section through a printed portion illustrating a notionally desired arrangement of applying this Overlap Method, in which the black layer 50 of width 501 overlaps by 2T beyond each outer edge of the white layer 30 and the design color layer 40 having a desired perceived color width 160. FIG. 3B illustrates an example to print the notional arrangement of FIG. 3A, the printed design color layer 40 and background white layer 30 being misaligned. Ink colors are typically designed to be printed on a white substrate and the desired color rendering will only be seen over the perceived color width 60 in which the design color layer 40 is superimposed over the white layer 30. The area of white exposed will lighten the perceived color and the design color layer 40 will be rendered less or not visible where it overlaps directly onto the black layer 50.
The prior art “Lateral Combination Method” of PCT/GB96/02600 illustrated in FIGS. 1C, 1D, 4A and 4B provides for the design color layer 40 to be seen in lateral combination with the white layer 30 by which is overlapped. In the notional arrangement of FIG. 4A, the outer edges of the design color layer 40 are 2T inside the width of the white layer 30. This ensures design color layer 40 will not overlap onto the black layer 50. A standard combined edge width of 4T of white is always seen in lateral combination with a standard width 401 of design color layer 40, thus enabling a method of printing to achieve uniform color rendering. The outer edges of white layer 30 are nominally 2T inside the outer edges of black layer 50, ensuring that the white layer 30 does not overlap the black layer 50. FIG. 4B illustrates an example printing of such an arrangement, in which the black layer 50 and design color layer 40 have been printed T to the left of their intended position and white layer 30 has been printed T to the right of its intended position but the criteria for consistent color rendering are maintained. This Lateral Combination Method has the disadvantage that there are always white edges visible throughout the panel and therefore design colors need to be adjusted to allow for this “whitening” of the printed design color ink. Another disadvantage is that very dark design layer colors, including black, cannot be produced by a consistent application of this method.
The prior art “Through Combination Method” of PCT/GB96/02600 illustrated in notional cross-section FIG. 5A requires the design color layer 40 to be translucent and to be seen in combination with the white layer beneath it. The notionally desired arrangement is for the outer edges of the white layer 30 to be 4T inside the outer edges of the black layer 50 and the outer edges of the design color layer 40 to be 2T inside the outer edges of the black layer 50. This ensures that the design color layer 40 covers the white layer 30 but does not overlap the underlying black layer 50. Consistent desired color rendering is achieved over the desired perceived color width 160, because where the design color layer 40 overlaps onto the black edge strips of black layer 50, the design color inks are rendered virtually invisible by the underlying black layer 50. However much the layers move relatively to each other within the maximum tolerance of plus or minus T, a uniform desired perceived color width 160 will always be achieved, as illustrated in FIG. 5B, in which black layer 50 and white layer 30 have moved by T to the left and design color layer 40 has moved by T to the right. This “Through Combination Method” has found wide commercial application in the UK and USA, for example in the production of panels according to GB 2 188 873  GB 2 165 292 as outdoor advertisements on the windows of payphone kiosks and retail establishments.
However, in the production of such panels, in order to achieve the required opacity of a white background layer 30, it has been found necessary to print several layers of white or to introduce one or more layers of silver in between the white and the black layers, both well known prior art methods of improving the perceived whiteness and opacity of a white layer. A notional arrangement of FIG. 5C in which two white layers 30 are superimposed leads in reality to the two layers 30 being offset, as illustrated in FIG. 5D, causing variation in the perceived color of design color layer 40. A notional arrangement of the “Through Combination Method” illustrated in FIG. 6A, in which a silver layer 70 of the same width as the white layer 30 is interposed between the white layer 30 and black layer 50 also causes problems of lack of registration. An actual printed portion, for example as illustrated in FIG. 6B, typically results in the layers being moved relative to each other by up to the maximum tolerance T, resulting in part of the design color layer 40 being superimposed on the white layer 30 on one edge with no underlying silver layer 70 and a part of the design color layer 40 being superimposed on silver layer 70 where there is no intervening white layer 30. Both of these parts of the design color layer 40 will tend to appear darker than with the intended construction of design color layer 40 over white layer 30 over silver layer 70 over black layer 50. Overlapping of the silver layer beyond the edges of the white layer can be prevented by adopting the notional arrangement of FIG. 7A, in which the outer edges of silver layer 70 are 2T inside the outer edges of white layer 30. However this is an arrangement and will result in an arrangement, such as FIG. 7B, in which there will still be a difference in perceived color depending on whether the design color layer 40 only has a white layer 30 between it and the black layer 50 or the desired arrangement of design color layer 40 over white layer 30 over silver layer 70 over black layer 50. For example, where a design requires an area of white, this will always be “less white” than if white layer 30 was underlain by silver layer 70 throughout the width of the white layer 30. Another disadvantage of the “Through Combination Method” is that to achieve consistent color rendering, the design color inks are required to be translucent, in order for them to be rendered substantially invisible when overlapping directly onto the black layer. While multi-color process inks, such as the 4 color process inks of cyan, magenta, yellow and black, are typically translucent, many ‘spot’ or ‘line’ colors of a single hue, intensity and greytone are opaque or sufficiently opaque to be clearly visible against a black background, especially darker ‘spot’ colors and metallic ‘spot’ colors, such as silver and gold. While ink manufacturers typically provide ‘spot’ colors in differing degrees of translucency, there is less choice and flexibility with a limitation of translucent inks. Silver, which is typically a particularly opaque ink and is used as an opacity ‘barrier’ background of white, as already described, is also a common color in advertising and corporate identity products, for example in the red, white and silver corporate identity of the Coca Cola company. Additionally, it is not possible to obtain ceramic inks that are sufficiently translucent for the Through Combination Method over the full range of desired colors. Another type of ink which is not effectively obscured by an underlying black layer is retro-reflective ink, typically comprising half-silvered glass micro-spheres which cause incident light to be reflected back along substantially the same light path. Retro-reflective ink is therefore clearly visible at night when illuminated by car headlights or other light source directed from the position of an observer. The ‘Through Combination Method’ is deficient in the manufacture of all such products incorporating design color layer inks which are not translucent or otherwise are clearly visible over a black layer.